G. Spina

The electron-nucleus dipolar coupling in slow rotating systems. 2. The effect of g anisotropy and hyperfine coupling when S = 12 and I = 32

Abstract An equation has been derived for the field dependence of nuclear T1−1 enhancement due to dipolar coupling with an electron of spin 1 2 which takes into consideration g anisotropy under slow rotation conditions. In analogy with the fast rotating systems, g anisotropy effects are found to be relatively small. An equation has also been derived for the S = 1 2 system coupled with a nuclear spin vector I = 3 2 under isotropic coupling conditions. The effect of coupling on the nuclear T1−1 values is dramatic for coupling constant A values larger than ℏτs−1, where τs is the electronic relaxation time. We have derived an analytical solution also for the case A∥ ≠ 0, A⊥ = 0 and I = 3 2 , while for actual cases with A∥ ≠ 0, A⊥ ≠ 0 numerical solutions are given. They are shown to account for previously unexplained experimental data. As an example, the fitting of the field dependence of water proton T1−1 values in solutions of the copper(II)-containing enzyme superoxide dismutase is presented. The experimental curves, including their temperature dependence, are nicely reproduced by using the actual ESR parameters. It is also stressed that for magnetic field values > 5 MHz, i.e., gβeH ⪢ A∥, A⊥, the simple Solomon equation is adequate for fitting the data.

The electron-nucleus dipolar coupling. The effect of zero-field splitting of an S = 32 manifold

Abstract An equation is derived to describe the water 1H T1− NMR values when water is interacting with high-spin cobalt(II) with S = 3 2 under slow rotation conditions. In particular the effect of the static zero-field splitting of the S manifold is taken into consideration. Comparison with the experimental data on the pseudotetrahedral cobalt(II)-substituted human carbonic anhydrase I is highly encouraging.

Dynamics of tin nuclei in alkyltin(IV)–deoxyribonucleic acid condensates by variable-temperature tin-119 Mössbauer spectroscopy

The dynamics of tin nuclei in the condensates SnR2(DNA monomer)2 and SnR3(DNA monomer)(R = Me or Et), freeze-dried, has been investigated by variable-temperature 119Sn Mossbauer spectroscopy. Linear functions In (At/A77.3)(T), In farel,abs(T) and 〈x2〉(T)(At= total area under the resonant peaks, fa the relative and the absolute estimates of Lamb-Mossbauer factors, and 〈x2〈 the mean-square displacements of the Mossbauer nucleus extracted from farel and faabs respectively) have been found at T 77.3 K, which indicate harmonic motions and the lack of phase transitions. The latter is also suggested by the temperature-invariant hyperfine parameters, isomer shift, nuclear quadrupole splitting (ΔE) and peak widths. From the slopes of the functions In At(T) and In farel(T), the dynamics of tin in alkyltin(IV)–DNA condensates is found to be analogous to that in organotin(IV) salts and complexes, on the assumption of effective vibrating masses, corresponding to molecular groups. The coincidence between farel,abs, as well as the related 〈x2〉, data, indicates that the negative charge on the DNA backbone phosphodiester groups is fully neutralized by alkyltin(IV) cations in SnR2(DNA monomer)2(R = Me or Et) as well as in SnEt3(DNA monomer), while only partially in SnMe3(DNA monomer) and in SnMe2(DNA monomer)2 obtained by standard procedures for DNA condensation. From the magnitude of the functions, as well as of the Debye temperatures, on fingerprint criteria, SnIVR2 moieties are assumed to bridge phosphodiester groups in toroidal condensates through interstrand bonding, while SnIVR3 would be appended to the double helix. Motions would involve SnR2(mononucleotide)2 and SnR3(mononucleotide) units as the effective vibrating masses. Two tin co-ordination sites occur for SnIVR2 moieties at the DNA surface, both trans-octahedral, and a single trigonal-bipyramidal site for SnIVR3, the organometal moieties being co-ordinated by phosphodiester and water oxygen atoms, according to ΔE rationalization by point-charge model structure simulations, as well as to Mossbauer–Zeeman spectra of the SnIVEt2– and SnIVEt3–DNA condensates.

Pair distribution function analysis and Mössbauer study of defects in microwave-hydrothermal LiFePO 4

Olivine-type LiFePO4 is nowadays one of the most important cathode materials of choice for high-energy lithium ion batteries. Its intrinsic defectivity, and chiefly the so-called lithium ironanti-site, is one of the most critical issues when envisaging electrochemical applications. This paper reports a combined diffractometric (Synchrotron Radiation XRD with Rietveld and PDF analyses) and spectroscopic (Mossbauer) approach able to give a thorough characterization of the material defectivity. Such analytical procedure has been applied to a sample prepared following an innovative microwave-assisted hydrothermal synthesis route that, in a few minutes, allowed us to obtain a well crystallized material. PDF analysis, which is applied for the first time to this type of battery material, reveals the presence of disorder possibly due to Li/Fe exchange or to a local symmetry lowering. A 5% amount of iron on the lithium site has been detected both by PDF as well as by Mossbauer spectroscopy, which revealed a small percentage of Fe3+ on the regular sites.

The electron-nucleus dipolar coupling in slow rotating systems. 3. The effect of isotropic exchange coupling of the electron spin with a second paramagnetic center

Abstract The solvent proton NMRD of a solution containing bis(ethylenediamine)copper(II) and hexacyanoferrate(III) has been measured at magnetic fields between 0.01 and 60 MHz. The two metal ions are known to be weakly magnetically coupled. The NMRD dispersion is similar to that expected on the basis of the simple Solomons approach although the values are sensibly smaller than those observed in solutions containing bis(ethylenediamine)copper(II) and diamagnetic hexacyanocobaltate(III). A theoretical treatment of the nucleus-unpaired electrons coupling, the latter under isotropic magnetic coupling conditions, is capable of accounting for the observed pattern.

Synthesis and Characterization of Amorphous 3Fe2 O 3 ⋅ 2 P 2 O 5 ⋅ 10 H 2 O and Its Electrode Performance in Lithium Batteries

Amorphous 3Fe 2 O 3 .2P 2 O 5 .10H 2 O was prepared by oxidation of iron(II) phosphate, obtained by spontaneous precipitation from iron(II) and phosphate aqueous solutions, by heating in air at 100°C. The material was characterized by thermal analysis, Mossbauer spectroscopy, X-ray powder diffraction, and scanning electron micrograph analysis. The material, tested as cathode in a nonaqueous lithium cell, exhibited a specific capacity of about 140 mAh g -1 at a current density of 25 mA g -1 . The utilization was reduced to about 76% by a tenfold increase of the discharge current. It showed an excellent cyclability. More than 1000 cycles were performed at about 50% of depth of discharge, with a capacity fade lower than 0.025%.

Study of the spin dynamics in an iron cluster nanomagnet by means of Mössbauer spectroscopy

The spin dynamics of a cluster of four iron (III) ions characterized by a spin ground state of 5 and Ising anisotropy has been investigated by means of Mossbauer spectroscopy from 10 to 80 K. For , spectra display a partially relaxed magnetic structure, while for higher temperatures, spectra are wholly relaxed. The trend of the probabilities of transition per unit time derived from the spectrum fittings is typical of an Orbach process or of a coupling with vibration modes in a narrow frequency range. According to an order-of-magnitude estimate, the relaxation mechanism should consist of modulations of the exchange interaction due to atom vibrations.

Fe-Doping-Induced Magnetism in Nano-Hydroxyapatites

Doping of biocompatible nanomaterials with magnetic phases is currently one of the most promising strategies for the development of advanced magnetic biomaterials. However, especially in the case of iron-doped magnetic hydroxyapatites, it is not clear if the magnetic features come merely from the magnetic phases/ions used as dopants or from complex mechanisms involving interactions at the nanoscale. Here, we report an extensive chemical-physical and magnetic investigation of three hydroxyapatite nanocrystals doped with different iron species and containing small or no amounts of maghemite as a secondary phase. The association of several investigation techniques such as X-ray absorption spectroscopy, Mössbauer, magnetometry, and TEM allowed us to determine that the unusual magnetic properties of Fe2+/3+-doped hydroxyapatites (FeHA) occur by a synergy of two different phenomena: i.e., (i) interacting superparamagnetism due to the interplay between iron-doped apatite and iron oxide nanoparticles as well as to the occurrence of dipolar interactions and (ii) interacting paramagnetism due to Fe3+ ions present in the superficial hydrated layer of the apatite nanophase and, to a lesser extent, paramagnetism due to isolated Fe3+ ions in the apatite lattice. We also show that a major player in the activation of the above phenomena is the oxidation of Fe2+ into Fe3+, as induced by the synthesis process, and their consequent specific positioning in the FeHA structure.

Mossbauer spectra of ferrihydride: superferromagnetic interactions and anisotropy local energy

The Mossbauer spectra of a sample of synthetic ferrihydrite have been investigated in the 26-220 K temperature range in order to identify the mechanism giving rise to the relaxation times. The results show that the temperature dependence of the relaxation times is in accordance with the Vogel-Fulcher law. This indicates that the magnetic interaction between the crystallites is significant. From the temperature dependence of the mean magnetic field under TB the authors also argue for a strong correlation between the spatial orientations of neighbouring clusters.

Analysis of velocity noise in Mössbauer experiments

Abstract We present a method to analyse velocity noise, as a part of a study on the behaviour of a new Mossbauer cryostat cooled by a closed cycle refrigerator. In the hypothesis of a stationary velocity noise on the sample during a Mossbauer measurement, we find that the resulting spectrum is the convolution of the noiseless spectrum with the velocity noise probability density (VNPD). In the light of this fact we follow Fourier Analysis to obtain the VNPD. This method is tested using spectra taken with a velocity noise generator. We also use the obtained VNPD to partially recover a heavily jammed spectrum.